This invention relates to weighing apparatus and more particularly to weighing apparatus utilizing multiple load cells.
Many weighing applications require the use of multiple load cells in a single scale or in a number of associated scales. For example, a heavy capacity scale for weighing truck or railroad cars requires multiple load cells. Each load cell provides an analog signal proportional to the portion of the load borne by that load cell. Strain gages connected in a wheatstone bridge configuration often provide the analog signal. In heavy capacity applications the load is distributed over usually at least four load cells and some applications may require sixteen or more load cells. The sum of the load cell output signals must be obtained to provide a signal representative of the total load. The usual technique for summing the signals from the analog cells has been to connect the outputs in parallel to provide a single analog output signal representative of the total weight applied to the scale.
The weighing accuracy of multiple load cell scales depends not only on the accuracy of the individual cells but also on the mechanical and electrical interaction among them. Since the load cells usually have different sensitivities to applied loads the total scale output is usually dependent upon the position of the weight on the scale. The outputs of the individual cells must, therefore, be compensated or adjusted so that the total scale output remains substantially the same for a given load no matter where on the scale it is positioned. Such load position compensation has usually been accomplished by connecting sensitivity reducing resistors in the wheatstone bridge circuit of the individual load cells, usually across the output of the bridge circuit. U.S. Pat. Nos. 4,261,195, to Lockery, 4,574,899 to Griffen and 4,556,115 to Lockery address the problem of load position compensation in multi-load cell scales.
Despite the solutions to the load position problem proposed in the above-identified patents, there remain problems associated with the connection of multiple analog load cells in parallel. The load cells when electrically connected together are interactive, so that a load cell will perform differently when tested alone than when tested with other load cells in a scale. The interactivity of the load cells connected together substantially complicates the problem of load position compensation by connection of resistors to individual load cells. A large number of iterations or repetitions may be required to arrive at the proper value of compensating resistor because a value initially determined to be appropriate for a particular load cell must be adjusted when compensating resistors are connected to other load cells. That adjustment can then require adjustment of the other compensating resistors, and so on.
When the analog circuits of load cells are connected together they are essentially impossible to monitor individually. Thus, "trouble shooting" or repair of a scale can require disassembly of the electrical circuits in order to test the load cells individually and find the defective one. Further, when a load cell is replaced for any reason the scale often requires recompensation for load position. A known test weight is required to accomplish this recompensation. For large scales in particular this is a time consuming procedure and the known weight often inconvenient to obtain.
Recently there has appeared the so-called "digital load cell" in which an analog-to-digital converter and microprocessor are dedicated to a single load cell. The electronic circuits are mounted on a printed circuit board connected directly to the load responsive spring element, or counterforce, of the load cell. Temperature, creep and linearity errors of the individual load cell have been compensated by digital techniques.